164 INVERTEBRATE MORPHOLOGY. 



over or in front of the resophagus, while the other arches 

 from one gauglionic mass to the other over the proboscis 

 sheath. Each ganglion is bilobed, the smaller posterior 

 lobe being in some cases united to the larger one by a rela- 

 tively thin baud of nerve-tissue so that it appears to be 

 almost independent. From the larger lobe of each side a 

 nerve-cord passes towards the posterior end of the body, 

 where the two may unite to form an arch passing over the 

 posterior part of the intestine. In addition to these a third 

 nerve originating from the commissure passing over the pro- 

 boscis sheath and running backwards in the median dorsal 

 line is frequently present as well as, in some forms, another 

 nerve running along the dorsal wall of the proboscis sheath, 

 to which it sends branches. In many forms, such as Cerebrat- 

 ulus, a fine plexus of nerve-tissue, lying between the muscle 

 layers of the body-wall, unites the three main nerve-cords, 

 some of the strands of the plexus being sometimes larger 

 than the others and forming circular commissures between the 

 nerve-cords ; in Tetrastemma and Amphipoms, for example, 

 these circular commissures may be strongly developed and 

 have an almost metameric arrangement, the general plexus 

 being in such cases wanting. 



Eyes (Fig. 84, oc) are present in some forms occasionally 

 in Considerable numbers, but are frequently wanting, and oto- 

 cysts occur but seldom. The lateral ciliated grooves which 

 occur 011 the sides of the head of some Khabdoccela reach in 

 the Nemerteans a high development (eg), in some forms, e.g. 

 Cerebratulus and Tetrastemma, becoming ciliated funnels of some 

 length, whose inner ends are imbedded in the substance of the 

 posterior lobe of the brain. An olfactory function has been 

 assigned to these organs, though some authors have consid- 

 ered them mainly respiratory. 



The excretory system consists of a longitudinal canal on 

 each side in the anterior portion of the body, sometimes re- 

 placed by a network of canals, which opens to the exterior by 

 one or more ducts leading to pores situated on the margin of 

 the body. In some cases these lateral ducts and the pores 

 may be quite numerous and, like the intestinal pouches and the 

 circular nerve-commissures, may have a somewhat metameric 



